1. Field of the Invention
The present invention relates to a novel compound that exhibits a liquid-crystal phase (hereinafter this may be referred to as “liquid-crystal compound”), and to a liquid-crystal composition and an optical material containing the compound. In particular, the invention relates to the liquid-crystal compound of such that its birefringence is large, the angle between the helical axis direction of the helical structure of the liquid-crystal phase of the compound and the major axis direction of the liquid-crystal molecule is large and the temperature range in which the compound exhibits a smectic phase is broad, and also relates to a liquid-crystal composition containing the compound.
2. Description of the Related Art
A liquid-crystal compound is roughly grouped into nematic, smectic and discotic liquid crystals from the molecular morphology and the molecular arrangement thereof. Of those, the smectic liquid crystal has a layered structure and is characterized in that it has a high-order structure. It is known that the smectic liquid crystal which exhibits a helical structure has useful optical properties of birefringence, optical rotation, polarization separation, diffraction, selective reflection, etc.
An optical film that expresses anisotropy has been proposed, which is produced by forming a smectic liquid crystal having such properties into a thin film in a mode of coating or the like followed by solidifying it (JP-A 2000-239402 and JP-A 2003-277754). The physical properties of liquid crystals that contribute to the properties of the optical film include the birefringence, the angle between the helical axis direction of the helical structure of the liquid-crystal phase of the compound and the major axis direction of the liquid-crystal molecule, the temperature range in which the liquid crystal exhibits a smectic phase, and the easiness in having a helical structure. Heretofore, however, no one knows a smectic liquid crystal which has a large birefringence and has a large angle to be formed by the helical axis direction of the helical structure of the liquid-crystal phase of the compound and the major axis direction of the liquid-crystal molecule, and no one could attain an optical filter having satisfactory properties. Accordingly, it is earnestly desired to develop a liquid-crystal compound having such properties.